the obama administration supports the challenge to the arizona law. and today's arguments on the heels of another case that could roll back a key portion of the voting rights act of 1965. for more on today's arguments, we turn as always to marcia coyle of the "national law journal." she was in the courtroom this morning, and is back with us tonight. so the outcome, marcia, of this could actually tip the federal-state balance on who gets to govern how we vote. >> that's true, gwen. the question before the justices is where do you draw the line between who has the authority to regulate elections. the election clause of the constitution actually gives authority to both. but where is the line when one crosses or goes too far than the other does? so that's the issue before the court. it wasn't clear today that it's going to be an easy line to find. >> ifill: the reason why this arizona law exists is because arizona officials say there's a problem involving illegal or undocumented immigrants registering to vote fraudulently. is there any evidence... did the